1. Field of the Invention
The present invention relates to a display, and more particularly, to an organic electroluminescent (EL) device and a method for fabricating the same.
2. Discussion of the Related Art
Organic EL devices, also called organic light emitting diodes (LEDs), are becoming very popular because of their possible application to flat panel displays (FPDs). They are extremely thin, matrix-addressable and can be operated at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional characteristics suitable for next generation FPDs such as little dependence on viewing angle and good device-formability on flexible substrates among other things. Organic LEDs differ fundamentally from conventional inorganic LEDs. While the charge transfer in inorganics is band-like in nature and the electron-hole recombination results in the interband emission of light, organic films are generally characterized by the low-mobility activated hopping transport and the emission is excitonic. Organic EL devices are also substantially different from conventional inorganic EL devices, especially in that organic EL devices can be operated at low DC voltages.
A substantial amount of research has been directed toward the efficiency improvement and color control of organic LEDs. The efficiency of some organic EL devices has now been demonstrated to be close to its theoretical limit and certainly adequate for many commercial applications. Moreover the color control is probably not limiting for many potential applications. In light of this, we believe that the outlook for commercial applications is excellent for organic EL devices. Their performance is quite satisfactory for many applications. It is valuable to think in terms of specific products and manufacturing techniques for the commercialization of organic EL devices. Consideration of the specific applications leads us to believe that more work on manufacturability, uniformity, reliability, and systems issues is required to commercialize organic EL devices.
Pixellation or patterning, especially of electroluminescent and second electrode materials, is one of the key issues to be solved before the commercialization of organic EL devices. The use of many conventional pixellation techniques is precluded due to the nature of organic materials which are extremely vulnerable to the attack by most solvents. Various efforts have been made to utilize shadowing effects for the film patterning. One approach was introduced by C. Tang et al. in U.S. Pat. No. 5,294,870, wherein partition walls were used as a means to give a shadowing effect. Therein each stripe of the second electrode is supposed to be separated by depositing the second electrode material onto the transparent support with relatively high partition walls at an oblique angle with the surface normal. The idea may be conceivable, but is not suitable at all for mass production. First of all, it is impossible to have a vapor source which may set the direction of traveling metallic vapor at a constant angle with the surface normal over the whole panel surface. Whether one has either a point source or a large area source, the angle would vary with the position of pixel on the transparent support because of the diverging nature of vapor. One may be able to obtain a certain degree of separation with a complicate arrangement of source and support, but only at the sacrifice of production field. More practical processes have been proposed by K. Nagayama et al. in U.S. Pat. No. 5,701,055 wherein electrically insulating, ramparts, like a bunch of tiny umbrellas, serve as a means to shadow the incoming vapor. An advantage of U.S. Pat. No. 5,701,055 over U.S. Pat. No. 5,294,870 is that depositions do not need to be made at an oblique angle, which markedly widens the process window.
FIGS. 1axcx9c1d illustrate typical process steps proposed in U.S. Pat. No. 5,701,055. Referring to FIG. 1a, an array of anode stripes 2 is formed of typically indium tin oxide on a transparent substrate 1, and insulating patterns 3 are formed thereon. As shown in FIG. 1b, next, each of ramparts 4 is formed to fit into the small gap between a pair of the insulating patterns 3. The rampart 4 with a pair of overhangs 8 serves an important role to shadow the incoming vapor. When organic materials and a second electrode material are vapor deposited, the films are formed on top of the ramparts as well as on the surface between the ramparts, as seen in FIG. 1c. The important point is that the films are not formed on the side surface of the ramparts due to a shadowing effect, which ensures the electrical insulation between any two adjacent pixels. After the formation of an electroluminescent multilayer 5 and a second electrode 6 on the first electrode 2 in succession, an encapsulation layer 7 is formed, as in FIG. 1d. In summary, the background art of U.S. Pat. No. 5,701,055 appears to be an effective way to pixellate an organic electroluminescent display panel.
However, the background art EL device has several drawbacks as follows.
(1) While a variety of ramparts with the different shape is shown in U.S. Pat. No. 5,701,055, the most typical one looks like that shown in FIG. 1b. Since this type of rampart 4 easily causes short-circuits between the first and second electrodes at the edges of the second electrode stripe, it is often necessary to form additional insulating patterns 3. But it is not necessary to fabricate them in the present invention.
(2) Ramparts are often formed of photoresist and the most important parts of them are the overhangs 8 which actually shadow the incoming vapor. It is quite a delicate process to make ramparts with good overhangs. The present invention provides ramparts or partition walls the production of which can be much simplified.
(3) Ramparts remain standing on the panel surface even after the completion of the device fabrication. A part of rampart itself or films on top of them may fall down to damage the device locally during the subsequent fabrication processes. To avoid this kind of potential problems, it is recommended to have a stable structure: the shape of trapezoid in the present invention is structurally more stable compared to the inverted trapezoid of the related art.
Accordingly, the present invention is directed to an organic electroluminescent device and a method for fabricating the same that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.
An object of the present invention is to provide an organic electroluminescent device and a method for fabricating the same, in which adequate pixellation can be achieved using simple and stable partition walls.
Another object of this invention lies in that the stable and solid feature of the partition walls employed in the invention can make a contribution towards the enhanced long-term stability of the electroluminescent device fabricated thereby.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the stricture particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages in accordance with the purpose of the present invention as embodied and broadly described the electroluminescent device includes a pluralist of pixels each with a first electrode, an organic luminescence multilayer, a second electrode, and successively prepared protection layers. For the purpose of pixellation, the device also has an array of partition walls with a trapezoidal structure made of an electrically insulating material. To achieve the electrical isolation between two adjacent pixels, the panel surface with a multilayer structure is subjected to a dry etching process.
In the other aspect of this invention, there is provided a method for fabrication an electroluminescent device, including the steps of (1) forming a plurality of first electrode stripes at fixed intervals on a transparent substrate, (2) forming an array of partition walls with a trapezoidal structure made of an electrically insulating material, (3) laminating an organic a electroluminescent multilayer and a second electrode layer in succession on the entire surface including on top of the partition walls, and (4) subjecting the panel surface to a dry etching process in order to remove a part of the films, especially the second electrode layer formed on top of the partition walls, thereby electrically isolating two adjacent pixels.